For traditional oxygen-implanted SOI, it may be desirable to make an SOI wafer and then build devices on the SOI. Current processes for making silicon-on-insulator substrate layers may involve using oxygen ion implant (“SiMOX”). SiMOX requires a high dose oxygen implant into a substrate, such as a silicon substrate, to build a sufficiently thick buried oxide (“BOX”) layer. However, converting the whole wafer surface all at once to an SOI layer often requires diffusion of significant quantities of oxygen through the top silicon surface in order to make a sufficiently high quality buried oxide.
Such oxygen diffusion requires an annealing process (also referred to as an oxidation process), which often occurs for a relatively long time at a relative high temperature. The annealing process turns the implanted layer into a true silicon dioxide, returns the top of the silicon to a defect-free state, and diffuses additional oxygen atoms through the SOI layer into the BOX layer. Additional oxygen is often used in the annealing process to consume small silicon particles left behind during the BOX layer formation. As this through-diffusion of oxygen competes unfavorably with oxidation of the top silicon surface (and the SOI-BOX interface), the process is often inefficient, and may require very long processing times at very high temperatures. Further, the top-surface oxidation may consume large portions of silicon available for the SOI layer.
After fabrication of these SOI substrates, processing generally continues employing conventional semiconductor integrated circuit fabrication techniques. An example of such processing includes forming a pad oxide and/or a pad nitride on the SOI. A mask is then applied to the pad oxide and/or pad nitride layers and the structure is etched. The mask is removed, and the resulting structure is annealed or oxidized to cure the portion of the SOI layer exposed by the etching. Additional processes, such as isolating SOI structures by filling spaces between devices with an electrically isolating material, such as silicon dioxide, may also occur.
The high cost of fabricating SOI SiMOX wafers is often a major cost hurdle for advanced semiconductor manufacturing. Current SOI manufacturing for silicon integrated circuits programs often has to pass along the higher costs to customers (e.g., keeping bulk silicon technology for everyday applications), or use substrates produced using bonding techniques. Bonding, however, also may be expensive. If high performance SOI is to become mainstream, cost reductions may be important.